1. Field of the Invention
The present invention relates a technique of a coloration producing material, especially relates to a coloration producing material capable of reversibly coloring-decoloring by an external stimulus, a composition containing the coloration producing material, and an optical element and a method for optical modulation using the composition. The coloration producing material of the present invention relates to a material which can be widely used as a functional material in such applications as the display of an image, a recording material, an optical element capable of controlling the amount of light transmission such as a light-adjusting element, a light-adjusting glass and so on, and a sensor.
2. Description of the Related Art
Traditionally, various coloration producing materials and the technologies therefor are known which are capable of reversibly presenting a color or a color density and which are designed for use in displays, recording, light adjustment, and sensors. Examples include an electrochromic material capable of reversibly coloring-decoloring using an electrical means, a photochromic material capable of reversibly coloring-decoloring using light, a thermochromic material capable of reversibly coloring-decoloring using heat, a material comprising a leuco dye combined with a color former and a color subtracter capable of reversibly coloring-decoloring the dye (Japanese Patent Application Laid-Open (JP-A) No. 2-188,294 and Japanese Patent Application Publication (JP-B) No. 7-65,037), a so-called guest-host liquid crystal comprising a dichroic dye and a liquid crystal and a display element using the liquid crystal, a method using the selective reflection of a cholesteric liquid crystal, and a method using the birefringence modulation of a liquid crystal (wavelength dispersion).
Also known is a technology wherein color display is performed by controlling the amount of light transmission or light scattering by using a polymer gel material (which is called a stimulus-responsive polymer gel) capable of reversibly causing a volume change (i.e., swelling and contraction) through a change in the pH, a change in the ionic strength, adsorption of material, addition of a solvent, heat, light, an electric current, or an electric field. For example, JP-A No. 61-149,926 proposes an optical element comprising a composition composed of a combination of a polymer gel capable of absorbing-desorbing a liquid by the action of an electric field and a colored liquid composed of a liquid and a pigment dispersed therein wherein the change in shape of the polymer gel causes the colored liquid to move so that the display is performed. JP-A No. 61-151,625 proposes an element using a colored polymer gel wherein the swelling of the polymer gel causes the optical density to decrease and the contraction of the polymer gel produces the coloration. JP-A No. 62-925 proposes an element wherein the display is performed by the change in light scattering of a polymer gel capable of absorbing-desorbing a liquid in compliance with a change in temperature. JP-A No. 4-274,480 proposes an element using a polymer gel having a dye linked thereto wherein the change in volume of the polymer gel causes the optical density to vary so that the display is performed. Further, JP-A No. 9-160,081 proposes an element capable of changing the hue by the change in volume of a polymer gel by using the change in shape of a polymer gel adsorbed onto the surface of pigment fine particles or colored particles wherein a nearly white color is displayed when the polymer gel is swollen and the color of the pigment particles or colored particles is displayed when the polymer gel is contracted.
Problems associated with traditional coloration producing materials and technologies thereof are as follows. In the case of an electrochromic material, it is difficult to produce a full-color range; the durability is poor; and the coloration speed is slow. In the case of a photochromic material and a thermochromic material, it is difficult to produce a full-color range; the colored state cannot be maintained for a long period of time; and the durability is poor. In the case of a material comprising a leuco dye combined with a color former and a color subtracter capable of reversibly coloring-decoloring the dye, the stability at the time of coloration is poor; the durability is poor; and the color density is low, although this material is being studied as a reusable heat-sensitive recording paper.
In the case of a so-called guest-host liquid crystal comprising a dichroic dye and a liquid crystal and a display element using the liquid crystal, a method using the selective reflection of a cholesteric liquid crystal, and a method using the birefringence of a liquid crystal (wavelength dispersion), the contrast is low; the visual angle is narrow; and the element and the methods are expensive, although the element and the methods are being studied for use as a reflection display element.
Meanwhile, in the case of the technology wherein the display is performed by controlling the amount of light transmission or the light scattering by utilizing a stimulus-responsive polymer gel, for example, in the optical element comprising a composition composed of a combination of a polymer gel capable of absorbing-desorbing a liquid by the action of an electric field and a colored liquid, as described in JP-A No. 61-149,926, it is required that the colorant does not mingle in the polymer gel. In reality, however, this technology is not without the apprehension that the repeated use of the element causes the colored liquid to mingle in the polymer gel to thereby contaminate the polymer gel and decrease the contrast and is further associated with the problem that the response speed is slow because of the use of a relatively large polymer gel.
The element using a colored polymer gel wherein the swelling of the polymer gel causes the optical density to decrease and the contraction of the polymer gel produces the coloration, as described in JP-A No. 61-151,625, cannot provide a satisfactory contrast at the colorant concentration described in the present specification. In the case of the element wherein the display is performed by the change in light scattering of a polymer gel capable of absorbing-desorbing a liquid in compliance with the change in temperature, as described in JP-A No. 62-925, it is difficult to provide a display in color because of the use of light scattering and the contrast is low.
The element using a polymer gel having a dye linked thereto wherein the change in volume of the polymer gel causes the optical density to vary so that the display is performed, as described in JP-A No. 4-274,480, cannot provide a satisfactory contrast at the dye concentration described in the present specification and an attempt to increase the dye concentration so as to enhance the contrast is associated with the apprehension that the polymer gel may be structurally changed and the response characteristics may become poor. Another problem is that the durability such as light fastness is poor because of the use of the dye. The element utilizing the change in shape of a polymer gel as described in JP-A No. 9-160,081 presents the problems that the structure of the polymer gel is very complicated; the production of the polymer gel is very costly.